The invention relates to a method for quenching a preheated cokeable bulk material for producing coke. The quenching is achieved by means of a fluid flowing through the preheated bulk material. During the quenching the preheated bulk material is closed off against the atmosphere and the steam formed from the quenching liquid and, if necessary, surplus quenching liquid are drawn off, for example, from the bottom of a quenching chamber. However, surplus liquid is to be avoided.
According to a quenching process known from U.S. Pat. No. 3,959,083 (Goedde et al) the bulk material is treated over a certain period of time nearly uniformly, i.e., a constant quantity of liquid is supplied per unit of time. U.S. Pat. No. 3,959,,083 discloses that in a quenching chamber which is closed at the top there is a physical correlation among the water quantity supplied per unit of time, the vapor pressure and the temperature of vapor escaping at the bottom of the quenching chamber. However, this reference limits the quantity of quenching water supplied per unit of time by stopping the supply when the vapor temperature has been cooled down to an optimal temperature of 400.degree. C. or lower. A direct control of the quenching water supply as a function of the vapor pressure is not disclosed in this reference.
In other conventional quenching processes in which quenching vessels, for example, coke quenching cars are used which are open at the top, the liquid supply is substantially constant per unit of time.
Preheated coking or cokeable bulk material, has very different physical properties depending on the starting raw material coal, and the quality and type of the heat treatment. The quality of the heat treatment depends, for example, on the temperature, the heat capacity and the heat transfer characteristic as well as the heat conductivity and the granular structure of the starting bulk material. In practice there may occur substantial differences in the just mentioned properties depending on the type of starting materials and these differences may affect the quenching results in an undesired manner. Yet, those skilled in the art have accorded to these properties a subordinate significance in the coking or quenching process although these properties may be quantitatively determined. Heretofore the primary concern in the quenching operation was directed to obtaining an absolutely quenched loose material, which, at best, should not exceed a certain remainder moisture content, for example, a moisture content of the quenching liquid as disclosed in said U.S. Pat. No. 3,959,083. Except for stopping the quenching liquid supply as disclosed in U.S. Pat. No. 3,959,083 when a certain vapor temperature has been reached, it is generally customary to treat the preheated loose bulk material freely or rather in an uncontrolled manner with quenching liquid over the whole period of the quenching process.
If the bulk material is preheated cokeable or coking material, substantial variations occur in the properties of the starting material characteristics if the quality of the coal varies, for example, from one coal mine to another. Variations may also result if the operating duration of the furnace, that is the heat treatment, is changed. If the resulting changes in the physical characteristics of the bulk material are not taken into account during the quenching operation, the following disadvantages may arise.
The quenching liquid quantity supplied per unit of time at the beginning of the coke quenching operation may be too large so that the bulk material is initially quenched too much, whereby substantial thermal stresses may occur. Such thermal or heat stresses may cause an extensive destruction of the bulk material to such an extent that an undesirably high proportion of small grained coke and coal slack or breeze is produced.
Further, the water quantity supplied toward the end of the coking operation per unit of time may be too large when the water is supplied as taught in the prior art, whereby certain zones in the bulk material may have a moisture content different from that in other zones of the bulk material. Since the water supply is determined with reference to the zone of the bulk material which is quenched last, other zones of the bulk material receive, toward the end of the quenching operation, liquid quantities which cannot anymore completely evaporate so that it is necessary to provide collecting containers for the excess quenching liquid. Such collecting containers must be equipped with rather expensive purification or cleaning plants. This incomplete evaporation is apparently due to the well known Leidenfrost effect. According to this effect the water drops are insulated from the hot surface of the bulk material by a steam layer which enables the water drops to penetrate deep down into the body of the bulk material. Each drop only explodes when its inner vapor pressure exceeds the surface tension of the drop at a temperature slightly above 100.degree. C.
Further, in prior art quenching operations a relatively large proportion of liquid droplets are entrained by the quenching steam during the introduction of the quenching water. These droplets withdraw heat from the quenching steam when the droplets themselves evaporate. Accordingly, the temperature of the quenching steam drops so that the efficiency of a heat recovery plant connected in series with the quenching plant is not at all economical or such efficiency is too small to be economically significant.
In U.S. Pat. No. 3,959,083 the quenching container is closed at the top. Therefore, the steam pressure that is generated when the quenching liquid is introduced on top of the bulk material under the closed cover, drives the quenching liquid drops and steam down through the body of the bulk material toward the open grating on which the bulk material rests. In the above U.S. Pat. No. 3,959,083 the initial vapor or steam temperature measured at the grating should be 700.degree. C. if no water exits yet from the grating and the initial temperature of the heated bulk material is 1000.degree. C. If these conditions can be maintained throughout the quenching operation, an optimal heat recovery is possible from the quenching operation.
According to the present invention it has been discovered that these initial conditions or rather their equivalents should be maintained throughout the duration of the quenching operation. However, the maintaining of such optimal heat recovery conditions during the entire quenching operation requires that a plurality of parameters are taken into account as will be explained in more detail below. The significance of these parameters has not been recognized heretofore by those skilled in the art.